1. Field of the Invention
The invention relates to optical transmission, particularly to small-sized transmitter/receiver units.
2. Description of the Related Art
In many communication applications an optical transmission link, particularly an infrared link, is an advantageous way to realise a transmission link. In most cases the optical transmission link is realised with an optical transmission unit comprising a transmitter operating in the optical wavelength range and a receiver operating in the corresponding range. In this application the optical wavelength range means that range of the electromagnetic spectrum, which ranges from and comprising the ultraviolet region (UV) up to and comprising the infrared region (IR), whereby it also comprises the range of visible light.
A generally used standard in optical transmission is the so called IrDA standard, and infrared links according to this standard are used i.a. in different infrared remote control devices, in data communication between computers and printers, and generally in the data communication of small-sized portable equipment. The IrDA standard is so widely used that currently there are available very small modules realised in hybrid techniques and containing a transmitter and receiver as well as a preamplifier circuit, which are required for the infrared link. Typically a light emitting diode (LED) acts as the transmitter, and a PIN diode acts as the receiver.
The smallest prior art IrDA modules have a structure like that in the example shown in FIG. 1. In the prior art solutions the PIN diode 2, the LED 1 and the preamplifier circuit 3 are placed side by side in a row on a substrate 7, whereby the width of the module 5 will be essentially the width of this row. In prior art arrangements it is also common that the transmitter 1 or LED uses a separate lens 4 and that the receiver 2 or the PIN diode uses another separate lens 6. Further the module 5 comprises a casing 9. The transmitter 1, the receiver and the preamplifier 3 are connected with bonding wires to the conductor pattern on the substrate, whereby conductor patterns on the substrate 7 form the required electrical connections between the pins and the preamplifier circuit 3, the receiver 3 and the transmitter 1. The module pins 11 can also be a part of the substrate, or they can also form the whole substrate, whereby the signal processing circuit 3, the receiver 2 and the transmitter 1 are directly mounted on the pins 11.
There are substantial disadvantages related to the prior optical transmission modules. Even the smallest prior art optical transmission modules are often too wide and large to be used for instance in mobile communication means. Further, it is very cumbersome and expensive to acquire and install two different lenses separately for the receiver and the transmitter.
The object of the invention is to realise an optical transmission module, which is smaller than known prior art solutions.
These objects are attained by mounting the preamplifier circuit 3, the receiver 2 and the transmitter 1 at least partially on top of each other, whereby the structure occupies substantially less space in the sidewards direction than prior art solutions. In such a solution the transmitter 1 shadows the receiver, whereby the reception sensitivity decreases. However, typical LEDs are small compared to the sensitive PIN diode, so that the savings in the mounting area of the module attained by the solution according to the invention will be a greater advantage than the disadvantage due to the decreased reception sensitivity.
The optical transmission unit according to the invention is characterized by that, which is stated in the characterizing part of the independent claim directed to an optical transmission unit. The mobile station according to the invention is characterised by that, which is stated in the characterizing part of the independent claim directed to a mobile station. The dependent claims describe further advantageous embodiments of the invention.